import React, {  useState } from 'react'
import { useEffect } from 'react'

export default function App() {
  const [opacity, setOpacity] = useState(1);


  useEffect(() => {
    const opacityTimer = setTimeout(() => {
      opacity -= 0.1;
      if (opacity <= 0) {
        opacity = 1;
      }
      setOpacity(opacity);
    }, 100);

    return () => {
      clearInterval(opacityTimer);
    }
  }, []);



  return (
    <div>
      <h1 style={{opacity}}>ReactHook真好用</h1>
      
    </div>
  )
}
 